If your system is in a hard hang, then you cannot break into the debugger. If you enable the deadman feature, the system panics instead of hanging indefinitely. You can then use the kmdb(1) kernel debugger to analyze your problem.
The deadman feature checks every second whether the system clock is updating. If the system clock is not updating, then you are in an indefinite hang. If the system clock has not been updated for 50 seconds, the deadman feature induces a panic and puts you in the debugger.
Take the following steps to enable the deadman feature:
Make sure you are capturing crash images with dumpadm(1M).
Set the snooping variable in the /etc/system file. See the system(4) man page for information on the /etc/system file.
Reboot the system so that the /etc/system file is read again and the snooping setting takes effect.
Note that any zones on your system inherit the deadman setting as well.
If your system hangs while the deadman feature is enabled, you should see output similar to the following example on your console:
panic[cpu1]/thread=30018dd6cc0: deadman: timed out after 9 seconds of clock inactivity panic: entering debugger (continue to save dump)
Inside the debugger, use the ::cpuinfo command to investigate why the clock interrupt was not able to fire and advance the system time.